<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    div{
        width:100px;
        height:100px;
        background:red;
    }
    </style>
</head>
<body>
    <div id="tes" class="test"></div>
    <script>
      var test=document.getElementById("test") || document.getElementsByClassName("test")[0];
      console.log(test);
      test.onclick=function(){
          this.style.backgroundColor="green"
      }
      /*   && ,|| ,!
        结果返回boolean值
        &&两边都为true 结果为true
        ||只要一边为true，结果为true
        ！ 取反
       */
       var a=20;
       var b=30;
       var c=40;
       var d=50;
       console.log(a>b && c<d);
    </script>
</body>
</html>